The Stovebolt Site Forums The Shop Area Engine Shop 305 V-6 Carburetor and Choke issues

I have the 305 V-6 in my '68 GMC. My original Bendix-Stromberg carburetor is long gone due to serious mechanical problems and missing parts.

I have replaced it with the ubiquitous Rochester 2-barrel using a custom machined adapter plate. This Rochester was factory automatic choke, equipped with vacuum and temperature coil spring setup. Because the truck was originally manual choke, I have attempted to modify the Rochester by removing the aforementioned auto choke goodies and splicing in the original choke cable brackets. I'm sure that is where the trouble began.

I can't seem to figure out the idle with the engine cold and on full choke vs. idle with a hot engine and no choke.

With a cold engine started with full choke, I set the idle where it sounded right to me. Then allowed the truck to warm up, opened up the choke butterfly, and drove it a little. Everything seemed to run fine.

Driving down the road, thermostat open and engine good and hot, I step on the clutch to coast to a stop. The engine dies and will not maintain idle where I have it adjusted. It starts and runs fine, but only if I keep my foot on the gas.

I obviously know only enough about carburetors to get myself in trouble. Any help with this would be greatly appreciated.

1. Any ideas what the rpms should be for a full-choke cold start idle?

2. What would cause it to run with the choke open and engine cool, but die once the engine is hot?

3. Vacuum port (originally for the auto choke) was reassigned to run the vacuum advance on my distributor. Could this be a big problem?

Very Confused,
JC

I don't know about the Rochester 2-barrel, but most automatic chokes have a fast idle cam. When set correctly, the cam makes the engine idle around 1200 RPM until warm and you tap the accelerator pedal. Then it idles down to normal idle speed.

When you set the "butterfly flap" in the carb throat too the open position, do you have it "Fully" open? It should be about 25% closed when the engine is at idle, if it is open too far you will not have enough vacumn to suck the fuel in at idle.
The way I set it is: start the engine and bring it up to operating temperature, then adjust the choke cable, so that the engine will idle (you will see the butterfly valve in the carb throat will be just a little over center), replace the breather , then make the final air/fuel mixture adjustments.

Converting an autochoke carburetor to a manual choke carburetor is tricky; and even more so to operate from now on.

First, you have to make absolutely certain that the fast idle cam still functions normally.

Second, you MUST adjust the manunal choke such that it NEVER completely closes! This is important. Most manual choke carbs have a poppet relief valve in the choke plate whereas auto choke carburetors do not. The vacuum will partially open the choke immediately on an auto choke, and you no longer have this feature.

If you have the above correct, start the engine, and warm to normal operating temperature. NOW is the time to set the idle (hot, not cold). If you did the conversion correctly, the fast idle cam will take care of the cold idle for you.

Finally, each time you go to pull the choke cable when the engine is turned off, it is imperative that you depress the footfeed at least halfway. This is necessary to disengage the choke lockout found on auto choke carburetors.

Really of more importance, is what engine is the donor engine for the carburetor? When migrating an O.E. carburetor from one engine to another, best results will be found if the donor engine is within 3 percent displacement of the donee engine. Since you have a 305, 3 percent is approximately 9. Thus, you should be able to adapt a carburetor from an engine within the range of 296 to 314 CID.

305 Stromberg carburetors are not that difficult to find.

Jon.

Thanks CarbKing. I love the fact that your tagline/signature hits so close to home. I am obviously working with equipment that should never have been bolted on in the first place. I will follow your steps and attempt to get her tuned up.

I am not sure what the donor engine was for the Rochester. I bought it remanf. from the parts store several years back and I don't recall what make/model/engine it came from. I will look for markings on the carb and try to verify its original use.

One quick question. What and where is the fast idle cam?

Also, you say the Stromberg carbs are not too hard to find? My local parts stores look at me like a leper when I ask for 305 V-6 parts (most of the time). Any ideas where to find one?

Thank you for your wisdom!

Jacob - the fast idle cam is a eccentric metal wheel with steps and a hole close to the center. A shoulder screw through the hole fastens the cam to the body of the carburetor, and the cam is permitted to rotate around the screw. The cam will be fastened to the choke butterfly valve. Moving the choke butterfly will cause the cam to rotate. The curb idle screw fits against the steps on the cam. When the choke butterfly is closed the cam will be rotated in such a way that the curb idle screw fits against the highest point of the cam, thus opening the throttle for a "fast idle". As the choke butterfly opens, the cam rotates to lower and lower steps contacting the screw. This causes the fast idle to slow to a normal idle.

Jon.